Carbon fibers have found wide application as constituents of composite materials such as reinforced polymers and metals. Carbon fibers provide such composites with improved properties such as greater strength, higher electrical and thermal conductivity and toughness. Polymeric composites with carbon fibers are used to make parts for automobiles, airplanes, parts for electromagnetic shielding or for support for catalytic particles.
Several methods are known in the art for producing carbon fibers. A first method involves dehydrogenating and graphitizing organic polymer filaments by heating them in a suitable atmosphere to make continuous carbon fibers with diameters typically between 1 and 5 .mu.m.
A second method involves producing discontinuous carbon fiber segments by vaporizing a hydrocarbon and then with a carrier gas contacting the hydrocarbon vapor with a suitable metal catalyst. This type of carbon fiber is known as "vapor grown carbon fiber" or VGCF. Typical VGCF consists of fibers a few .mu.m in diameter with lengths ranging from a few microns to several centimeters. The catalyst can be either particulate or can be produced in the gas phase by decomposition of a suitable metal-containing precursor.
Baker and Harris in Chemistry and Physics of Carbon, Vol. 14, page 83 (1978) disclose forming VGCF carbon filaments by contacting ethylene or benzene vapor with cobalt at 1000.degree. K. However, these filaments have large diameters generally greater than 5 nm.
U.S. Pat. No. 4,663,230 discloses contacting a vapor such as benzene, ethylene, acetone, carbon monoxide or the like with a metal-containing particle (e.g. iron, cobalt or nickel) at an elevated temperature to form carbon fibers having a diameter of 3.5 to 70 nm.
Oberlin et al., in the J. of Crystal Growth, 32, p. 335 (1976) discloses a two step process for making carbon fibers. The first step involves pyrolysing a mixture of benzene and hydrogen at 1100.degree. C. to form primary carbon filaments having parallel carbon layers and then the second step involves depositing carbon on these filaments to thicken the filaments.
Carbon fibers comprising a small number of nested carbon tubes will have remarkable properties. Such fibers will have very high strength by virtue of the nature and regularity of their bonding and therefore will provide superior properties to composite materials. They can serve as catalytic surfaces that would confine species in an effectively 1-dimensional space. Arrays of such fibers might be used as filters or sieves. Iijima, in Nature 354, 56 (1991) shows such carbon fibers (nanotubes) with multiple concentric cylindrical shells of hexagonally bonded carbon atoms which are produced in the cathode deposit of a carbon arc generator run with a helium atmosphere of a few hundred Torr. These nanotube fibers have typical outside diameters greater than 2 to several tens of nm.
A still more desirable fiber is a fiber with a wall comprising a single layer or carbon atoms. These single atomic layer fibers could be used to assemble structures with low density and high surface to volume ratios, wires with extremely small diameters and solids with highly anisotropic properties. They also could be semiconducting or metallic depending on their helicity. These single atomic layer fibers could be used directly in assemblies or structures, or could serve as uniform "seed" substrates for growth of larger ordered structures.
Ajayan et at., in Nature, 358, 23 (Jul. 2, 1992) discloses a transmission electron microscope image which the authors speculate is an end-on view of a single isolated structure. However, the process method utilized by the authors does not result in carbon fibers having a wall comprising a single layer of carbon atoms.
Tsang et al., in Nature 362, 520 (Apr. 8, 1993) discloses forming a multilayered carbon fiber having a short end stub comprising a single layer of carbon atoms. Tsang formed this singular fiber by selectively oxidizing a great number of multilayered carbon fibers. Due to the oxidative nature of the process which simultaneously oxidizes both the circumference and the end of the fiber (e.g. the cap), this process is limited to forming a short single atom layer stubs on the end of multilayered fibers.
Therefore, there still is a need in the art for carbon fibers having a wall comprising a single layer of carbon atoms and a process for production of such carbon fibers.
It is therefore an object of the present invention to provide a carbon fiber having a wall comprising a single layer of carbon atoms.
It is another object of the present invention to provide a carbon fiber having a wall comprising a single layer of carbon atoms and a length greater than 100 nm.
It is another object of the present invention to provide a process for the production of carbon fibers having a wall comprising a single layer of carbon atoms.
Other objects and advantages will become apparent from the following disclosure.